Maybe a good way to begin to address the challenges and difficulties you both are facing is to agree to be monogamous for now?

I was with my partner for years and years before we discussed polyamory, and we were both ready for that by then. It works for us. We love each other very much and have a basically good, healthy and happy committed relationship. It is from this strength that polyamory is workable for us. And it seems to me that couples who are not already both inclined to poly, and capable of handling it well, should probably first be monogamous with one another for a year or two--at least. Once a strong connection is deepened by years of commitment to one another, the conversation about poly could -- perhaps -- be raised again.

Ask your partner if he will practice monogamy with you for a given period--say a year, or two. And tell him that you'll be very willing to open-mindedly and honestly discuss poly -- or open relationship, non-mongamy -- after that period. That could ease your fears while you develop and deepen your relationship.